1. Field of the Invention
This invention relates to a float valve for use in a fuel tank, placed in an upper portion of the fuel tank and connected to an air outflow channel, for allowing air to flow from the fuel tank when refueled, and for stopping the outflow of air when the level of fuel rises to a filled-up position.
2. Description of the Related Art
Conventional float valves placed in a fuel tank are disclosed in, for example, Japanese Unexamined Patent Publication No. 5-185850/1993 and Japanese Examined Utility Model Publication No. 4-39061/1992 and have a structure as illustrated in FIG. 1 (Prior Art).
The float valve V0 has a float 5 inserted in a closed-bottomed cylindrical casing 1 which is fixed to a top wall 9A of a fuel tank 8A. The casing 1 has a ceiling wall 2, a bottom wall 3 located under the ceiling wall 2, and a side wall 4 connecting the ceiling wall 2 with the bottom wall 3. A communicating hole 2a providing communication with an outflow channel 7A is bored through ceiling wall 2.
A through hole 3a and an air hole 4a are formed in the bottom wall 3 and the side wall 4, respectively, so as to permit fuel F and air A, respectively, to flow into then casing 1.
A valve portion 5a formed on the top of the float 5 is capable of blocking the flow of air to outflow channel 7A. A compression coil spring 6 is located between the float 5 and the bottom wall 3 of the casing 1.
The compression spring 6 is a biasing means for biasing the float 5 to block the communicating hole 2a when the vehicle is inclined or overturned. Namely, when the vehicle is overturned, the communicating hole 2a is positioned under the float 5. At that time, the spring constant of the spring 6 is set so that the sum of the mass of the float 5 and the force of the spring 6 is larger than the buoyant force of the float 5. Nevertheless, the spring constant of the spring 6 is set in a range so that the float 5 is not raised and the communication hole 2a is not blocked before the level of the fuel F rises when the vehicle is upright. The sum of the buoyant force of the float 5 and the force of the spring 6 is larger than the mass of the float 5 when the float 5 is immersed in fuel F.
It is usual in such a float valve V0 that if the level of the fuel F contained in the fuel tank 8A rises, the fuel F flows into the casing 1 by way of the through hole 3a, and displaced air and fuel vapors enter through the air hole 4a. As a result, the float 5 rises. Thus the valve portion 5a blocks the communicating hole 2a, and the fuel F can be prevented from flowing from the tank 8A. Air which flows through the communicating hole 2a and the air outflow channel 7A is sent to a canister (not shown).
In the case of a fuel tank 8A using the conventional float valve V0, when refueling, a large amount of the fuel F is put into the fuel tank 8A in a short time. This causes the following problem.
Specifically, when refueling, the air A flows rapidly into the casing 1 from the air hole 4a. Thus the velocity of the flow of the air A flowing into the air outflow channel 7A becomes high. Consequently, the flow of the air A provides lift and operates to lift the float 5. Further, when the fuel F flows into the casing 1 from the through hole 3a the float 5 is further lifted by the buoyant force due to the fuel F. Thus the flow of the air A sometimes causes the valve portion 5a to block the communicating hole 2a before the surface or level of the fuel F rises to a filled-up tank position LF. This inappropriate valve closing operation is apt to occur especially when the buoyant force owing to the fuel F is exerted on the float 5, because the downward force on the float 5 is small (namely, the value obtained by subtracting the buoyant force and the spring force from the mass is small).
Further, when the valve closes inappropriately, the air A normally discharged from the fuel tank 8A remains in the tank 8A. Thus the mechanism for automatically stopping a fuel injection gun injecting fuel into the fuel tank 8A is actuated when the fuel level at the fuel inlet rises. Consequently, an inappropriate valve closing operation interferes with filling up the fuel tank.
Moreover, such inappropriate valve closing is liable to occur where the refueling speed is high, namely, where the amount of fuel flow is great, because the resulting flow of the air A is thus also great.